In one type of nuclear reactor power plant, liquid metal, such as liquid sodium, is used for the coolant which is heated by the reactor core. The liquid metal is circulated within the reactor by a plurality of annular linear-flow electromagnetic induction pumps which propel the liquid metal using electromagnetic force.
A typical electromagnetic pump includes an annular flow channel or duct surrounded by a column of alternately vertically stacked annular electrical stator coils and magnetic stator iron cores collectively forming a stator. Electrical current carried by the coils induces magnetic flux which is channeled through the cores in a predetermined direction for inductively propelling the liquid metal through the flow channel.
An electromagnetic pump may have solely a single stator surrounding the outside of the flow channel for propelling the liquid metal therethrough. Or, an electromagnetic pump may have double stators including an outer stator surrounding the flow channel, and an inner stator disposed radially inwardly of the flow channel. Each stator is similarly configured with alternating layers of stator coils supported by stator cores. Since the double stator electromagnetic pump is effective for generating magnetic flux both radially inwardly and radially outwardly of the flow channel, it is typically more effective for propelling the liquid metal axially through the flow channel.
In both types of pumps, the stator coils typically include a plurality of turns or windings of copper electrical conductors disposed coaxially about the longitudinal centerline axis of the pump. The alternating stacks of stator coils are supported on respective stator cores, with each stator core including a plurality of circumferentially adjoining iron laminations. The laminations are typically flat, sheet metal components which are relatively thin for reducing eddy current losses therein. Since the laminations are a constant thickness and extend radially outwardly from the pump centerline axis, they can tightly abut each other at their radially inner ends, but at their radially outer ends a larger circumference is provided than the laminations have the capability of filling. Accordingly, the laminations are typically configured in groups having a constant thickness in the circumferential direction, with generally V-shaped gaps being formed between adjacent ones of the groups.
Since the laminations are individual elements and subject to vibration during operation of the pump, they must be suitably clamped together to restrain vibration thereof. For example, the laminations may be fixedly joined to each other by a suitable adhesive, or may be joined together in the groups by suitable mechanical fasteners, or may also be joined together using a band or wire extending circumferentially around the outer ends of the groups, which band is placed in tension to clamp together the several groups in a complete ring as disclosed for example in U.S. Pat. No. 4,882,514--Brynsvold et al, which is assigned to the present assignee.
In one design being developed, the band is a substantially 360.degree. ring having a single split defining two ends which are generally L-shaped with radially outwardly extending flanges through which a nut and bolt fastener is provided to clamp together the band ends to place the band in tension for clamping together the several lamination groups. In tests of this clamping band design, the ability to maintain adequate tightness in the band for suitably clamping together the lamination groups was obtained only after repeated shakedown cycles of tightening the fastener, heating and maintaining the so assembled stator core at elevated temperature for about a week, followed by cool-down to a hot standby condition and repeating this cycle.
The tests suggested that the initial band tension was being lost in cycling of the stator core due in part to the eccentrically loaded fastener; relatively high friction between the band and the outer edges of the individual laminations which affected the ability to maintain uniform tension around the circumference of the band and a uniform conformance of the band to the perimeter contour of the lamination groups around the circumference of the band; and thermal expansion of the lamination groups which exceeded expansion of the clamping band which loosened the band upon thermal contraction of the lamination groups, with the initial circular configuration of the band upon assembly being distorted by the differential thermal expansion between the lamination groups and the band.
The ability to maintain a suitable tension in the band during operation of the stator core in the electromagnetic pump is required to maintain structural integrity of the stator core during its useful lifetime while restraining vibration thereof. Since the stator cores will necessarily undergo cycling from relatively low to relatively high temperatures during operation in response to cyclical operation of the reactor core itself, a suitable band clamp which can maintain adequate tension therein and clamping together of the lamination groups is required.